The diameter of a Josephson junction (junction) in a superconductor integrated circuit (IC) should be the smallest definable feature in order to obtain maximum circuit performance. This diameter should be limited by only the resolution of the lithography tool and the etch tool. However, the diameter of the junction in a conventional superconductor IC is limited by the diameter (or surface area) of its contact. More specifically, in a conventional superconductor IC, the junction diameter must be greater than the diameter of its contact in order to prevent an unwanted short circuit between wire layers. As a result, the minimum junction diameter is determined by the minimum contact diameter plus approximately two times the alignment tolerance from the lithography tool.
For example, the minimum junction diameter in a conventional superconductor IC must be approximately 50% larger than the contact diameter for high yield in accordance with alignment tolerances of existing lithography tools. As a result, a 1.5 μm diameter junction requires a contact diameter of no greater than 1.0 μm to permit an alignment error of +/−0.25 μm. However, even if the alignment tolerances were significantly improved, the junction diameter would still have to be greater than the contact diameter. This is because a junction contact that is larger in diameter than the junction diameter in a conventional superconductor IC can result in unwanted short circuits between wire layers.
If the relationship between the junction diameter and the contact diameter were significantly decoupled, the junction diameter would become the minimum size feature. The junction diameter would then be reduced to 1.0 μm or smaller in accordance with generally accepted design rules. This would lead to an increase in the current density of the critical current by a factor of 2.25. Because the circuit speed scales with the square root of the current density, the circuit speed would be increased by approximately 50%.
Accordingly, an object of the present invention is to provide a superconductor IC fabrication method for producing a superconductor IC with a minimum size diameter junction.
A further object of the present invention is to provide a superconductor IC fabrication method requiring limited additional fabrication.
A further object of the present invention is to provide a superconductor IC device that prevents unwanted short circuits between wiring layers.